1. Field of the Invention
This invention relates to vehicular powertrains, and more particularly to improvements in vehicular powertrains which facilitate low engine rpm which high torque output capability, and with respect to internal combustion engines, provides high fuel economy and low emissions of exhaust gas pollutants.
2. Description of the Prior Art
In recent years, the need for controlling certain components of the exhaust gases emitted from internal combustion engines, especially automotive engines, has been recognized. Unfortunately, the methods used for reduction of exhaust emissions, most notably nitrogen oxides, also often cause a significant reduction in fuel economy.
Since the formation of nitrogen oxides depends strongly on combustion temperature, changes that reduce this temperature have been used effectively as a control. These changes usually include adjustments such as reduced compression ratios and relatively retarded spark timing. In contrast to the adjustments just mentioned, the combustion of moderately lean air-fuel mixtures can improve fuel economy while also lowering combustion temperature and consequently nitrogen oxides emissions. Although these benefits of lean mixture strength are well-known, the accompanying problems of the difficulty of spark ignition of lean mixtures and the greatly increased sensitivity to variations in mixture strength are also well-known.
The stratified charge engine embodies the most advanced method of emissions control yet to be used in high volume automotive production. The stratification of the air-fuel mixture, in contrast to the relatively homogeneous mixture used in more conventional engines, provides a solution to the problem of spark ignition of mixtures having a relatively lean overall air-fuel ratio. This solution is simply to locate the spark plug in an area of the combustion chamber occupied by a relatively rich, easily ignitable portion of the air-fuel mixture. Additionally, refined carburetion and intake manifolding have solved the driveability problem by reducing cycle-to-cycle and cylinder-to-cylinder variations in air-fuel ratio. As a result, low exhaust emissions have been achieved with efficiency and driveability comparable to that of the conventional automotive engines used before the adoption of emissions control equipment.
Another approach which controls the legally defined exhaust gas pollutants with no significant fuel economy penalty is treatment of the exhaust gases with a catalytic converter. However, in spite of widespread usage on automobiles now being produced, the continued use of the catalytic converter is questionable because of greatly increased toxic emissions of sulfuric acid.
Beyond the recent advances which eliminate the fuel economy penalty associated with strict control of exhaust emissions, methods of significant fuel economy increases, still in conjunction with tightly controlled emissions, are presently needed. The Stirling cycle engine promises this, but more development work is needed and cost may remain a major obstacle. Although the stratified charge engine is another possibility, a significant change is required in comparison to the stratified charge engine previously mentioned. The previously mentioned engine uses throttling of its combustion intake air to control engine torque to the desired value, but it is the pumping loss associated with this throttling that must be eliminated to realize a fuel economy advantage. Since spark ignition of utra-lean air-fuel mixtures is possible in a stratified charge engine, torque can be controlled by varying the air-fuel ratio, ultra-lean mixtures producing limited torque values largely without the necessity for throttling. In other words, engine torque can be limited by restricting only fuel flow, rather than by restricting both fuel and air flow as in a throttled engine. However, the required apparatus for appropriately varying air-fuel ratio is expensive and, more importantly, the extremely low combustion temperatures occurring at ultra-lean mixture strengths are insufficient for thorough oxidation of gaseous exhaust emission of hydrocarbons.
In conclusion, the prior art includes no practical way of controlling exhaust pollutants to a low level while simultaneously increasing fuel economy a substantial amount in comparison to the automotive engines and powertrains used previous to the adoption of emissions control equipment.